Method for producing an ultra high strength galvannealed steel sheet and obtained galvannealed steel sheet

ABSTRACT

A method for producing a coated steel sheet having a tensile strength TS of at least 1450 MPa and a total elongation TE of at least 17% includes the successive steps of providing a cold rolled steel sheet made of a steel having a chemical composition comprising, in weight %: 0.34%≤C≤0.45%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0%&lt;Cr≤0.7%, 0%≤Mo≤0.3%, 0.10%≤Al≤0.7%, and optionally 0%≤Nb≤0.05%, the remainder being Fe and unavoidable impurities, annealing the cold-rolled steel sheet at an annealing temperature AT higher than the Ac3 transformation point of the steel, quenching the annealed steel sheet by cooling it down to a quenching temperature QT lower than the Ms transformation point of the steel and comprised between 150° C. and 250° C., and reheating the quenched steel sheet to a partitioning temperature PT between 350° C. and 450° C. and maintaining the steel sheet at the partitioning temperature PT for a partitioning time Pt of at least 80 s, and coating the steel sheet by galvannealing, with an alloying temperature GAT comprised between 470° C. and 520° C.

This is a continuation of U.S. application Ser. No. 16/067,055 which hasa filing date of Jun. 28, 2018 and which is a national stage ofPCT/IB2015/060026 filed December 29, the entire disclosures of which arehereby incorporated by reference herein.

The present disclosure concerns the manufacture of a high strengthgalvannealed steel sheet having an improved tensile strength and animproved total elongation, and a galvannealed steel sheet obtained bythis method.

BACKGROUND

To manufacture various equipment such as parts of body structuralmembers and body panels for automotive vehicles, it is now usual to usesheets made of DP (dual phase) steels multi-phase, complex phase ormartensitic steels.

For example, a high strength multi-phase may include abainite-martensitic structure with/without some retained austenite andcontains about 0.2% of C, about 2% of Mn, about 1.5% of Si which wouldresult in a yield strength of about 750 MPa, a tensile strength of about980 MPa, and a total elongation of about 10%. These sheets are producedon continuous annealing line by quenching from an annealing temperaturehigher than Ac3 transformation point, down to an overaging temperatureabove Ms Transformation point and maintaining the sheet at thetemperature for a given time. Optionally, the sheet is galvanized orgalvannealed.

SUMMARY

To reduce the weight of the automotive parts in order to improve theirfuel efficiency in view of the global environmental conservation it isdesirable to have sheets having improved strength-ductility balance. Butsuch sheets must also have a good formability.

Besides, it is desirable to produce a galvannealed steel sheet, sincegalvannealing provides an improved weldability and a high-corrosionresistance after spot welding and stamping.

In this respect, it is desirable to provide a galvannealed steel sheethaving a tensile strength TS of at least 1450 MPa and a total elongationTE of at least 17%. These properties are measured according to ISOstandard ISO 6892-1, published in October 2009. It must be emphasizedthat, due to differences in the methods of measure, in particular due todifferences in the size of the specimen used, the values of the totalelongation according to the ISO standard are very different, inparticular lower, than the values of the total elongation according tothe JIS Z 2201-05 standard. Furthermore, it is desirable to produce thegalvannealed sheets with a manufacturing method which is robust, i.e.such that variations in the method parameters do not lead to importantvariations of the mechanical properties obtained

Therefore, the purpose of the present disclosure is to provide such asheet and a robust method to produce it.

For this purpose, a method is provided for producing a galvannealedsteel sheet, the method comprising the successive steps of:

-   -   providing a cold rolled steel sheet made of a steel having a        chemical composition comprising, in weight %:        -   0.34%≤C≤0.45%        -   1.50%≤Mn≤2.30%        -   1.50≤Si≤2.40%        -   0%<Cr≤0.7%        -   0%≤Mo≤0.3%        -   0.10%≤Al≤0.7%,        -   optionally 0%≤Nb≤0.05%, and            the remainder being Fe and unavoidable impurities,    -   annealing the cold-rolled steel sheet at an annealing        temperature AT higher than the Ac3 transformation point of the        steel,    -   quenching the annealed steel sheet by cooling it down to a        quenching temperature QT lower than the Ms transformation point        of the steel and comprised between 150° C. and 250° C.,    -   reheating the quenched steel sheet to a partitioning temperature        PT between 350° C. and 450° C. and maintaining the steel sheet        at the partitioning temperature PT for a partitioning time Pt of        at least 80 s,    -   coating the steel sheet by hot dip coating in a zinc bath        followed by galvannealing, with an alloying temperature GAT        comprised between 470° C. and 520° C.

According to other advantageous aspects of the present disclosure, themethod further comprises one or more of the following features,considered alone or according to any technically possible combination:

-   -   during quenching, the annealed steel sheet is cooled down to the        quenching temperature QT at a cooling rate enough to avoid        ferrite formation upon cooling, in order to obtain a quenched        steel sheet having a structure consisting of martensite and        austenite,    -   said cooling rate is higher than or equal to 20° C./s,    -   the quenching temperature is between 200° C. and 230° C.,    -   the partitioning time Pt is comprised between 100 s and 300 s,    -   the annealing temperature AT is comprised between 870° C. and        930° C.,    -   the alloying temperature GAT is comprised between 480° C. and        500° C.,    -   the steel sheet is maintained at the alloying temperature GAT        for a time GAt comprised between 5 s and 15 s,    -   the composition of the steel is such that Al≤0.30%,    -   the composition of the steel is such that 0.15%≤Al,    -   the composition of the steel is such that 0.03%≤Nb≤0.05%,    -   said galvannealed steel sheet presents a tensile strength TS of        at least 1450 MPa and a total elongation TE of at least 17%.

A galvannealed steel sheet is also provided made of a steel having achemical composition comprising, in weight %:

-   -   0.34%≤C≤0.45%    -   1.50%≤Mn≤2.30%    -   1.50≤Si≤2.40%    -   0%<Cr≤0.7%    -   0%≤Mo≤0.3%    -   0.10%≤Al≤0.7%,    -   optionally 0%≤Nb≤0.05%, and        the remainder being Fe and unavoidable impurities, the structure        of the steel consisting of between 50% and 70% of martensite,        residual austenite, and bainite.

According to other advantageous aspects of the present disclosure, thegalvannealed steel sheet comprises one or more of the followingfeatures, considered alone or according to any technically possiblecombination:

-   -   the composition of the steel is such that Al≤0.30%,    -   the composition of the steel is such that 0.15%≤Al,    -   the composition of the steel is such that 0.03%≤Nb≤0.05%,    -   the retained austenite has a C content comprised between 0.9%        and 1.2%,    -   said galvannealed steel sheet presents a tensile strength TS of        at least 1450 MPa and a total elongation TE of at least 17%.

DETAILED DESCRIPTION

The present disclosure will now be described in details but withoutintroducing limitations.

According to the present disclosure, the sheet is obtained by heattreating a hot-rolled and preferably cold rolled steel sheet made ofsteel having a chemical composition comprising, in weight %:

-   -   0.34% to 0.45% of carbon to ensure a satisfactory strength and        to improve the stability of the retained austenite, which is        necessary to obtain a sufficient elongation. If the carbon        content is above 0.45%, the hot rolled sheet is too hard to cold        roll and the weldability is insufficient.    -   1.50% to 2.40% of silicon in order to stabilize the austenite,        to provide a solid solution strengthening and to delay the        formation of carbides during partitioning with appropriate        procedures to prevent the formation of silicon oxides at the        surface of the sheet which would be detrimental to the        coatability. Preferably, the silicon content is higher than or        equal to 1.80%. Preferably, the silicon content is lower than or        equal to 2.20%.    -   1.50% to 2.30% of manganese. The minimum content is defined to        have a sufficient hardenability in order to obtain a        microstructure containing at least 50% of martensite, and a        tensile strength at least 1450 MPa. The maximum is defined to        avoid having segregation issues which are detrimental to the        ductility.    -   0% to 0.3% of molybdenum and 0% to 0.7% of chromium to increase        the hardenability and to stabilize the retained austenite, in        order to strongly reduce austenite decomposition during        partitioning. The absolute zero value is excluded due to        residual amounts. According to an embodiment, the composition        comprises from 0% to 0.5% of chromium. Preferably, the        molybdenum content is comprised between 0.07% and 0.20%, and the        chromium content is preferably comprised between 0.25% and        0.45%.    -   0.10% to 0.7% of aluminum. Aluminum is added to obtain a high        level of elongation as well as a good strength-ductility        balance, and to increase the robustness of the manufacturing        method, in particular to increase the stability of the        mechanical properties obtained when the quenching temperature        and the partitioning time vary. The maximum aluminum content of        0.7% is defined to prevent an increase of the Ac3 transformation        point to a temperature which would render the annealing more        difficult. Preferably, the aluminum content is higher than or        equal to 0.15%, and/or lower than or equal to 0.30%, which        allows obtaining a total elongation TE of at least 17% and a        uniform elongation UE of at least 16%. Preferably, aluminum is        added at a late stage, after the deoxidation stage.

The remainder is iron and residual elements or unavoidable impuritiesresulting from the steelmaking. In this respect, Ni, Cu, V, Ti, B, S, Pand N at least are considered as residual elements which are unavoidableimpurities. Therefore, generally, their contents are less than 0.05% forNi, 0.05 for Cu, 0.007% for V, 0.001% for B, 0.005% for S, 0.02% for Pand 0.010% for N.

Addition of microalloy elements such as niobium from 0% to 0.05% and/ortitanium from 0% to 0.1% can be utilized to obtain the desiredmicrostructure and an optimal combination of product properties, inparticular an increased tensile strength. For example, Nb is added in anamount comprised between 0.03% and 0.05%.

Hot rolled sheet having can be produced in a known manner from thissteel.

As an example, a sheet having the above composition is heated to atemperature between 1200° C. and 1280° C., preferably about 1250° C.,hot-rolled with a finish rolling temperature preferably less than 850°C., then cooled and coiled at a temperature preferably comprised between500° C. and 730° C. The sheet is then cold-rolled.

After rolling, the sheet is pickled or cleaned, then heat treated andgalvannealed.

The heat treatment, which is preferably made on a continuous annealingand hot dip coating line, comprises the following successive steps:

-   -   annealing the cold rolled sheet at an annealing temperature AT        equal or higher than the Ac3 transformation point of the steel,        and preferably higher than Ac3+15° C., in order to obtain an        annealed steel sheet having a structure completely austenitic,        but less than 1000° C. in order not to coarsen too much the        austenitic grains. Generally, a temperature higher than 870° C.        is enough for the steel according to the present disclosure and        this temperature does not need to be higher than 930° C. Then        the steel sheet is maintained at this temperature i.e.        maintained between AT−5° C. and AT+10° C., for a time sufficient        to homogenize the temperature in the steel. Preferably, this        time is of more than 30 seconds but does not need to be more        than 300 seconds. To be heated to the annealing temperature, the        cold rolled steel sheet is, for example, first heated to a        temperature of about 600° C. at a heating rate typically below        20° C./s, for example below 10° C./s, then heated again to a        temperature of about 800° C. at a heating rate typically below        10° C./s, for example below 2° C./s, and eventually heated to        the annealing temperature at a heating rate below 5° C./s, for        example below 1.5° C./s. In this case, the sheet is maintained        at the annealing temperature AT for an annealing time At between        40 and 150 seconds.    -   quenching of the annealed sheet by it cooling down to a        quenching temperature QT lower than the Ms transformation point,        and comprised between 150° C. and 250° C. The annealed sheet is        cooled to the quenching temperature QT at a cooling rate enough        to avoid the formation of ferrite formation upon cooling.        Preferably, the cooling rate is comprised between 20° C./s and        50° C./s, for example higher than or equal to 25° C./s. The        quenching temperature QT and the cooling rate during quenching        are chosen so as to obtain a quenched sheet having a structure        consisting of martensite and austenite. The martensite and the        austenite contents in the quenched sheet are chosen so as to        allow obtaining, after the heat-treatment and the galvannealing,        a final structure consisting of 50% to 70% of martensite,        retained austenite, and bainite. If the quenching temperature QT        is lower than 150° C., the fraction of the partitioned        martensite in the final structure is too high to stabilize a        sufficient amount of retained austenite, so that the total        elongation does not reach 17%. Moreover, if the quenching        temperature QT is higher than 350° C., the fraction of        partitioned martensite is too low to obtain the desired tensile        strength. Preferably, the quenching temperature QT is comprised        between 200° C. and 230° C.    -   reheating the quenched sheet up to a partitioning temperature PT        comprised between 350° C. and 450° C. The heating rate is        preferably of at least 30° C./s.    -   maintaining the sheet at the partitioning temperature PT for a        partitioning time Pt of at least 80 s, for example comprised        between 80 s and 300 s, preferably at least 100 s. During the        partitioning step, the carbon is partitioned, i.e. diffuses from        the martensite into the austenite which is thus enriched in        carbon. The degree of partitioning increases with the duration        of the holding step. Thus, the holding duration Pt is chosen        sufficiently long to provide a partitioning as complete as        possible. However, a too long duration can cause the austenite        decomposition and too high partitioning of martensite and,        hence, a reduction in mechanical properties. Thus, the        partitioning time is limited so as to avoid as much as possible        the formation of ferrite.    -   hot-dip coating the sheet in a zinc bath followed by        galvannealing, at an alloying temperature GAT. The heating to        the alloying temperature is made preferably at a heating rate of        at least 20° C./s, preferably at least 30° C./s. Preferably, the        alloying temperature GAT is comprised between 470° C. and        520° C. Still preferably, the alloying temperature is lower than        or equal to 500° C. and/or higher than or equal to 480° C. The        sheet is maintained at the alloying temperature GAT for a time        GAt which is for example comprised between 5 s and 20 s,        preferably between 5 s and 15 s, for example between 8 s and 12        s.    -   cooling the galvannealed sheet down to the room temperature        after galvannealing. The cooling speed to the room temperature        is preferably between 3 and 20° C./s.

This heat-treatment and galvannealing allows obtaining a final structurei.e. after partitioning, galvannealing and cooling to the roomtemperature, consisting of martensite, with a surface fraction comprisedbetween 50% and 70%, retained austenite and bainite.

A fraction of martensite comprised between 50% and 70% allows obtaininga tensile strength of at least 1450 MPa.

Furthermore, this treatment allows obtaining an increased C content inthe retained austenite, which is of at least 0.9%, preferably of atleast 1.0%, and up to 1.2%.

With this heat-treatment, it is possible to obtain sheets having a yieldstrength of at least 900 MPa, a tensile strength of at least 1450 MPa, auniform elongation of at least 16% and a total elongation of at least17%.

As examples and comparison, it was manufactured sheets made of steelswhose compositions in weight % and critical temperatures such as Ac3 andMs are reported in table I.

TABLE I Ref C Mn Si Cr Mo Al Nb Ac3 Ms steel % % % % % % % ° C. ° C. I10.41 2.02 1.92 0.31 0.16 0.17  — 875 305 C1 0.38 1.98 1.93 0.51 0.0030.048 0.039 825 290

The underlined values are not according to the invention.

Several sheets were heat treated by annealing at a temperature TA for atime to of 80 s, quenching at a temperature QT at a cooling rate of 25°C./s, reheated to a partitioning temperature PT at a reheating rate of40° C./s and maintained at the partitioning temperature PT for apartitioning time Pt, then galvannealed at an alloying temperature GATfor a time GAt or 10 s, then cooled to room temperature at a coolingrate of 5° C./s.

The mechanical properties were measured in the transverse directionrelative to the direction of rolling. As it is well known in the art,the ductility level is slightly better in the direction of rolling thanin the transverse direction for such high strength steel. Measuredproperties are the yield strength YS, the tensile stress TS, the uniformelongation UE and the total elongation TE.

The conditions of treatment and the mechanical properties are reportedin Table II.

In these tables, AT is the annealing temperature, QT the quenchingtemperature, PT the partitioning temperature, Pt the partitioning time,and GAT is the alloying temperature.

TABLE II AT QT PT Pt GAT YS TS UE TE Example Steel ° C. ° C. ° C. s ° C.MPa MPa % % 1 I1 900 215 400 100 500 990 1479 16.5 22   2 I1 900 215 400200 500 950 1460 16.6 22.1 3 I1 900 215 400 300 500 1070 1450 16.4 21.54 I1 900 230 400 100 500 910 1460 18 23   5 I1 900 230 400 200 500 9501465 18.1 24   6 I1 900 230 400 300 500 1000 1450 16.6 22   7 C1 900 205400 50 500 1062 1548 14.7 16.5 8 C1 900 205 400 100 500 990 1561 14.316.5 9 C1 900 205 400 150 500 998 1581 12.7 14.3

Examples 1-6 show that with a steel having a composition according tothe invention, in particular comprising 0.17% Al, with a quenchingtemperature QT of 215° C. or 230° C., and a partitioning temperature PTof 400° C., a steel sheet having a high level of elongation and a goodstrength-ductility balance can be obtained. Indeed, the sheets ofexamples 1-6 all have a yield strength of at least 910 MPa, a tensilestrength of at least 1450 MPa, a uniform elongation UE of at least 16.5%and a total elongation TE of at least 17%, and even 21%.

The comparison of the mechanical properties of examples 1-6 further showthat the desired mechanical properties obtained are almost non sensitiveto the quenching temperature QT ranging from 215° C. to 230° C. and tothe partitioning time Pt when it is comprised between 100 s and 300 s.Thus, the properties obtained are very robust to variations of thequenching temperature and/or the partitioning time.

By comparison, the properties of examples 7-8, made of a steelcontaining 0.048% Al, are more sensitive to variations of thepartitioning time Pt.

What is claimed is:
 1. A method for producing a galvannealed steelsheet, the method comprising successive steps of: providing a coldrolled steel sheet made of a steel having a chemical compositionincluding, by weight: 0.34%≤C≤0.45% 1.50%≤Mn≤2.30% 1.50≤Si≤2.40%0%<Cr≤0.7% 0%≤Mo≤0.3% 0.10%≤Al≤0.7%, optionally 0%≤Nb≤0.05%, and aremainder, the remainder including Fe and unavoidable impurities;annealing the cold rolled steel sheet at an annealing temperature AThigher than an Ac3 transformation point of the steel; quenching thesteel sheet by cooling the steel sheet down to a quenching temperatureQT, lower than a Ms transformation point of the steel, between 150° C.and 250° C.; reheating the steel sheet to a partitioning temperature PTbetween 350° C. and 450° C.; maintaining the steel sheet at thepartitioning temperature PT for a partitioning time Pt of at least 80 s;and coating the steel sheet by hot dip coating in a zinc bath followedby galvannealing, with an alloying temperature GAT between 470° C. and520° C., the steel sheet being heated to the alloying temperature with aheating rate of at least 20° C./s.
 2. The method according to claim 1,wherein during the quenching step, the steel sheet is cooled down to thequenching temperature QT at a cooling rate to avoid ferrite formationupon cooling, and to obtain a quenched steel sheet having a structureconsisting of martensite and austenite.
 3. The method according to claim2, wherein the cooling rate is greater than or equal to 20° C./s.
 4. Themethod according to claim 1, wherein the quenching temperature isbetween 200° C. and 230° C.
 5. The method according to claim 1, whereinthe partitioning time Pt is between 100 s and 300 s.
 6. The methodaccording to claim 1, wherein the annealing temperature AT is between870° C. and 930° C.
 7. The method according to claim 1, wherein thealloying temperature GAT is between 480° C. and 500° C.
 8. The methodaccording to claim 1, wherein the steel sheet is maintained at thealloying temperature GAT for a time GAt between 5 s and 15 s.
 9. Themethod according to claim 1, wherein the chemical composition of thesteel includes, by weight, 0.10%≤Al≤0.30%.
 10. The method according toclaim 1, wherein the chemical composition of the steel includes, byweight, 0.15%≤Al≤0.7%.
 11. The method according to claim 1, wherein thechemical composition of the steel includes, by weight, 0.03%≤Nb≤0.05%.12. The method according to claim 1, wherein the galvannealed steelsheet has a tensile strength TS of at least 1450M/Pa and a totalelongation TE of at least 17%.
 13. The method according to claim 1,wherein providing the cold rolled steel sheet includes hot-rolling asteel sheet with a finish rolling temperature less than 850° C. thencold rolling.
 14. The method according to claim 1, wherein to be heatedto the annealing temperature, the cold rolled steel sheet is firstheated to a temperature of 600° C. at a heating rate below 20° C./s,then heated again to a temperature of 800° C. at a heating below 10°C./s and eventually heated to the annealing temperature at a heatingrate below 5° C./s.